Transformer Primary Fuse Size Calculator
Calculate optimal primary fuse protection for transformers according to NEC & IEC standards
Transformer Primary Fuse Calculator
Compliant with NEC Article 450 & IEC 60076 Standards
Calculation Results:
Primary Current: 0.00 A
Protection Factor: 0%
Calculated Fuse Size: 0.00 A
Recommended Fuse Size: 0 A
Standard Rating: -
How to Use the Transformer Primary Fuse Size Calculator
Step 1: Select Standard
- Choose NEC for North American installations
- Select IEC for international applications
- Different standards have varying protection factors
- NEC Article 450 provides specific guidelines
- IEC 60076 offers international standards
Step 2: Enter Transformer Data
- Input the transformer kVA rating from nameplate
- Enter primary voltage (line-to-line for 3-phase)
- Select single or three-phase configuration
- Choose appropriate fuse type for application
- Verify all data before calculation
Step 3: Interpret Results
- Review calculated primary current
- Check protection factor applied
- Note recommended standard fuse size
- Consider inrush current characteristics
- Verify compliance with local codes
How to Calculate Transformer Primary Fuse Size
Primary Current Calculation
The primary current is calculated using:
- Single Phase: I = (kVA × 1000) / V
- Three Phase: I = (kVA × 1000) / (√3 × V)
- Where V is the primary voltage
- Result gives full load current in amperes
Protection Factor Selection
NEC Article 450.3(B) specifies protection factors:
- ≤ 9A: 300% maximum protection
- 9A to 20A: 250% maximum protection
- > 20A: 125% maximum protection
- Time-delay fuses may use higher factors
Real-Life Calculation Example
Given: 75 kVA, 480V primary, three-phase transformer
Step 1: Calculate Primary Current
I = (75 × 1000) / (√3 × 480)
I = 75,000 / (1.732 × 480)
I = 75,000 / 831.36
Primary Current = 90.21 A
Step 2: Determine Protection Factor
Since 90.21A > 20A, use 125% factor (NEC 450.3(B))
Step 3: Calculate Fuse Size
Fuse Size = 90.21 × 1.25 = 112.76 A
Recommended Standard Fuse: 110A or 125A
Step 4: Final Selection
Choose 110A for tighter protection or 125A for better coordination
Consider transformer inrush current characteristics
Calculation Formulas
Single Phase Primary Current:
I = (kVA × 1000) / V
Three Phase Primary Current:
I = (kVA × 1000) / (√3 × V)
Fuse Size Calculation:
Fuse = I × Protection Factor
NEC Protection Factors
Important Considerations
- Consider transformer inrush current (5-8 times FLC)
- Time-delay fuses handle inrush better
- Verify local electrical code requirements
- Consider coordination with downstream protection
- Account for ambient temperature effects
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